The electrical contact leads of integrated circuits are often coated with a thin layer of solder, by plating or other techniques, so as to improve their connection and solderability characteristics. This classically has been done after the chip has been assembled and using hot dip processes which result in relatively thick solder coatings. A newer, more efficient process has been recently adopted, called pre-plating, in which the solder is deposited before assembly so as to increase productivity. However, the pre-plated solder layer is necessarily quite thin because of the stringent dimensional requirements and productivity requirements that the leads must meet. The lead base materials are usually copper or nickel alloys.
A severe problem arises with these new thin coatings, however, because industry practice requires severe quality control tests for integrated circuits which include subjecting the parts to high temperatures of 155-175 degrees centigrade for extended periods of many hours to test the integrity of the part. Also, the assembly process involves the application of heat to cure the molded DIPS. This additional heat sometimes affected the solderability of prior art pre-plated leads which did not become apparent until the part actually reached the end user who had difficulty in properly soldering the part into a circuit board. After an extensive investigatory program, it was discovered that a close control of the lead to tin ratio in the solder will eliminate later burnin induced solderability problems for thin layers of solder on copper, nickel, or other base materials that can form intermetallics with tin.